This invention relates generally to burners for burning a combustible gas comprised of a mixture of fuel gas and air. More particularly, the invention relates to a burner of the pre-mix type where the mixing of the fuel gas and air has occurred before the combustible gas reaches the burner.
Burners for burning a combustible gas find use in a wide variety of applications. One use is in hot air furnaces, where the burning gas heats air for the purpose of warming the interior of a building such as a house. In such a furnace, the burning gas and gases of combustion are combined with a heat exchanger, such as heat exchanger 50 in FIG. 1, while air to be heated passes over and around the exterior of the heat exchanger. Heat exchanger 50 is of the clamshell type and is typical of the heat exchangers found in residential warm air furnaces. Such a heat exchanger is manufactured by embossing two matching raised patterns into sheet metal and joining the embossed patterns together to form heat exchanger flue path 51. The joints are made so that flue path 51 is gas tight except for flue inlet 52 and flue outlet 53. The typical furnace has more than one heat exchanger, the number being dependent on the size and heat transfer characteristics of each individual heat exchanger and the desired furnace heating capacity. Other furnace structure isolates the flue inlets and outlets from the air to be heated. Other furnace designs use tubular heat exchangers such as heat exchanger 60 shown in FIG. 2. Heat exchanger 60 is functionally similar to heat exchanger 50 in that air to be heated passes around the exterior of flue 61 and the burning gas and gases of combustion are confined to the interior of the flue path between flue inlet 61 and flue outlet 62.
In the typical prior an hot air furnace, an inshot burner, such as burner 30 depicted schematically in FIG. 3, burns fuel gas and air to produce hot gases of combustion. Fuel gas is supplied to burner 30 through gas inlet 32. Air, introduced through primary air inlet 36, mixes with the fuel gas and burns, producing primary flame 33. Other air, known as secondary air, mixes with the unburned gas in primary flame 33 and produces secondary flame 34. The result is that the total length of flame from an inshot burner is relatively long. An inshot burner is positioned at the flue inlet, such as flue inlet 52 (FIG. 1) or flue inlet 62 (FIG. 2), of each heat exchanger in the furnace so that the flame projects into the heat exchanger flue.
The combustion of a fuel gas such as methane, particularly at very high temperatures can produce, as products of combustion, various oxides of nitrogen, collectively known as NO.sub.x. These oxides vent to the atmosphere with other combustion products. Limiting the concentration of NO.sub.x in desirable, as certain jurisdiction may place restrictions on NO.sub.x emissions. Furnaces sold in those jurisdictions must comply with very stringent emission standards.
Furnace designers have found that the use of pre-mix burners can greatly reduce NO.sub.x emissions. Unlike an inshot burner, where fuel gas and air mix in the burner, the fuel gas and air are mixed to form a combustible gas at a point in the fuel gas and air supply paths before reaching the pre-mix burner. FIG. 4 depicts schematically a typical prior art pre-mix burner. Burner 40 has burner body 41, combustible gas inlet 42 and flame holder 44. Flame holder 44 is perforated so that combustible gas can pass through the holder and burn as flames 49 slightly off its surface. In such a burner as burner 40, the flames, and thus the heat output, are concentrated in the immediate vicinity of the burner.
A pre-mix burner having physical and operating characteristics similar to burner 40 would not be suitable for use with a heat exchanger such as heat exchanger 50 or 60. The heat exchanger wall would necessarily be in close proximity to the burner and thus the concentration of the heat produced in the immediate vicinity of the burner would result in excessively high temperatures in the wall of the heat exchanger. Such high temperatures would increase surface temperatures of the surrounding heat exchanger and shorten the life of the heat exchanger. U.S. Pat. No. 4,960,102, issued 2 Oct. 1990 to Shellenberger, describes and depicts a furnace having a burner like burner 40. The furnace avoids the problem of excessive temperatures in the heat exchanger wall by constructing the wall to be sufficiently far from the burner that excessive temperatures do not occur.
The figures of U.S. Pat. No. 3,525,325, issued 25 Aug. 1970 to Perl, appear to disclose a gas flame burner having a concave flame holder but a close reading of the disclosure shows that the '325 burner is of the radiant infrared and not of the flame type.
Clamshell and tubular type furnace heat exchangers offer a number of operational, cost and manufacturing advantages. Large numbers are in use and they are still in production. What is needed is a burner of the pre-mix type, with its low NO.sub.x emission qualities, that can be used with a clamshell or tube type heat exchanger. Such a burner should not have combustion characteristics that would lead to excessive heat exchanger wall temperatures, even if the wall is in close proximity to the burner.